Transitions between B-DNA and Z-DNA: a dilemma

The intramolecular conformational change which occurs in double-stranded (dG-dC) oligonucleotides and polynucleotides in high salt solutions is currently assumed to be a transition between the presently accepted structures for B-DNA and Z-DNA. However, a serious dilemma results from the fact that these two structures are from topologically different families of helical duplexes. Mechanisms for interconversion between the two forms, under typical experimental conditions, are highly improbable from a physical-chemical viewpoint. The alternative possibility is that the structure of one of the forms in the transition is not as assumed. The resulting dilemma is addressed and alternative relationships between right- and left-handed DNA forms are discussed.